1. Field of the Invention
The present invention relates to a proton conductive solid polymer electrolyte to be used for a variety of electrochemical cells including, for example, a fuel cell and a hydrogen and oxygen generator for generating hydrogen and oxygen by electrolyzing water, and a method for producing the same.
2. Description of the Related Art
In the case of the fuel cell, for example, an electrolyte is interposed between an anode to which fuel gas containing hydrogen is supplied and a cathode to which oxygen-containing gas such as air is supplied. The electrolyte moves the hydrogen ion (proton), which is generated by ionizing hydrogen contained in the fuel gas on the anode, to the cathode. In other words, the electrolyte serves as a proton conductor in the fuel cell.
A material, which is obtained by humidifying a perfluorosulfonic acid polymer membrane with liquid water, is widely known as an example of the proton conductor which acts as the electrolyte of the fuel cell. The proton conductivity of the membrane lowers as the membrane is dried. For this reason, in order to maintain the power generation characteristics of the fuel cell, the membrane is prevented from being dried by keeping the fuel gas and/or the oxygen-containing gas contained with steam to continuously replenish the membrane with water, and supplying a cooling medium into the fuel cell to maintain the operation temperature at 80° to 90° C.
In recent years, a composite electrolyte comprising a base material of basic solid polymer such as polybenzimidazole, the base material impregnated with liquid inorganic strong acid such as phosphoric acid (see U.S. Pat. No. 5,525,436), and a composite electrolyte comprising a base material of meta-polyaniline and constructed in the same manner as described above (see Japanese Laid-Open Patent Publication No. 2001-160407) have been suggested.
The two types of the composite electrolytes described above have high proton conductivities even in a dried state. Therefore, it is unnecessary to use any humidifiers. Further, because the fuel cell can be operated at high temperatures, the cooling system may be small. Therefore, the fuel cell system can also be constructed simply and small in size.
As well-known, H2O is produced when the fuel cell is operated. When the fuel cell is operated at relatively low temperatures, for example, during the start-up operation or during the low output operation of the fuel cell, H2O is in a form of liquid water.
Meanwhile, phosphoric acid has an extremely high degree of liquid absorption. Therefore, when the composite electrolyte as described above is adopted as an electrolyte of the fuel cell, a quantity of the H2O which is produced by the operation of the fuel cell is absorbed by phosphoric acid. In particular, phosphoric acid absorbs the liquid during the low temperature operation such as the start-up operation and the low output operation. Therefore, the concentration of phosphoric acid is lowered, and the amount of phosphoric acid is increased. When the amount of phosphoric acid exceeds an amount which the base material of the composite electrolyte is capable of retaining, phosphoric acid begins to exude out of the composite electrolyte.
The exudate phosphoric acid is discharged outside of, the fuel cell via the fuel gas passage which supplies the fuel gas to the anode, and/or via the oxygen-containing gas passage which supplies the oxygen-containing gas to the cathode. Namely, phosphoric acid outflows, and the concentration of phosphoric acid remaining in the base material of the composite electrolyte is lowered. Therefore, the proton conductivity of the composite electrolyte is lowered. Consequently, the internal resistance of the fuel cell rises, deteriorating the power generation characteristics of the fuel cell.
In order to improve the water absorption of such phosphoric acid, the following suggestion has been made in U.S. Pat. No. 6,124,060 and Japanese Laid-Open Patent Publication No. 2000-38472. H of the —OH group possessed by phosphoric acid (OP(OH)3) is substituted with a functional group having phenyl group to produce substituted acid as illustrated in the following chemical formulas (1) and (2). In the chemical formulas (1) and (2), R represents H, alkyl group having a number of carbon atom or atoms of 1 to 5, halogen group, or nitro group.

A polymer having imidazole ring is impregnated with the substituted acid to form a composite electrolyte (proton conductive solid polymer electrolyte).
The substituted acid has a low degree of water absorption as compared with phosphoric acid. Therefore, it is possible to avoid the increase in amount of the substituted acid, and it is possible to avoid the decrease in concentration.
However, there is a great demand to obtain an electrolyte which has an excellent ability to retain the components that contribute to the proton conductivity even when the operation is performed under the condition in which a large amount of liquid water is produced, for example, during the operation at low temperatures as described above.
The substance or material, which exhibits proton conductivity, is exemplified by an acidic group-possessing organic compound bonded with a functional group (acidic group) which makes any bonded substance to be acidic. In order to lower the degree of elution of the acidic group-possessing organic compound into water, it is conceived that the acidic group-possessing organic compound molecules are polymerized with each other to produce an acidic group-possessing polymer.
In order for the proton conductivity to be substantially equivalent to one another even when the operation condition resides in low temperature or high temperature with low humidity, it is necessary to obtain a large mole number of the acidic group contained per gram of the acidic group-possessing polymer. Specifically, 9×10−4 mole of acidic group is contained in 1 g of the polymer of perfluorosulfonic acid, but an amount, which is several times the above, is required.
If, however, the mole number of the acidic group is increased, the acidic group-possessing polymer becomes water-soluble. Therefore, if the mole number of the acidic group is excessively increased, then the acidic group-possessing polymer is dissolved in liquid water produced, for example, in the low temperature operation, and the power generation characteristics of the fuel cell are deteriorated.
A method for suppressing the water solubility in the acidic group-possessing polymer is to blend the acidic group-possessing polymer and a basic polymer. In general, as reported by J. Kerres et al. in Solid State Ionics (1999), Vol. 125, pp. 243–249, such a blend polymer is obtained by blending the acidic group-possessing polymer and the basic polymer in an organic solvent.
However, if the same operation is performed by using an acidic group-possessing polymer having a large mole number of acidic group per gram, in order for the proton conductivity to be substantially equivalent to one another even when the operation condition resides in low temperature or high temperature with low humidity as described above, the gelation is caused by the acid-base interaction in the organic solvent. Therefore, the acidic group-possessing polymer and the basic polymer are not blended sufficiently.
As described above it is difficult to prepare a material which contains the basic polymer and the acidic group-possessing polymer having a large mole number of acidic group.